1. Field of the Invention
The invention relates to a self-propelled ground milling machine, in particular a road milling machine or road recycler, which has a machine frame supported by running gears and a working roller arranged on the machine frame in a roller housing, a hold-down device which is height-adjustable with respect to the traffic surface being arranged upstream of the working roller in the working direction. The invention further relates to a method for working on a traffic surface using a self-propelled ground milling machine, in particular a road milling machine or road recycler, in which the traffic surface is worked on using a working roller arranged in a roller housing, a hold-down device which is height-adjustable with respect to the traffic surface being arranged upstream of the working roller in the working direction.
2. Description of the Prior Art
Known self-propelled ground milling machines have a machine frame, which is supported by an undercarriage which has front and rear running gears, and a working device arranged on the machine frame for working on a traffic surface, for example for removing damaged traffic surfaces or for reconditioning existing traffic surfaces. The working device may comprise a milling and/or cutting roller. Lifting devices are assigned to the individual running gears of the ground milling machine, and can each be retracted and extended in such a way that the machine frame can be lowered and raised with respect to the ground surface. In a rear-loader road milling machine, the milled material is transported away, by a transport device, downstream of the machine in the working direction, where it can subsequently be loaded onto a subsequent transport vehicle, whilst in a front-loader road milling machine, the milled material is transported away via the front face.
In road milling machines, it is known for the working roller to be arranged in a roller housing, which is closed off upstream of the working roller in the working direction by a hold-down device which is height-adjustable with respect to the traffic surface and downstream of the working roller in the working direction by a height-adjustable stripper device. On each side of the roller housing, a height-adjustable edge protector may be located. During the milling process, the height of the hold-down device, stripper device and edge protector is usually set in such a way that the hold-down device and edge protector are positioned on the traffic surface, whilst the stripper device runs in the milled track downstream of the milling roller. During operation of the ground milling machine, the material comminuted by the working roller accumulates in the roller housing. The milled material can remain in the milled track if the stripper device is raised, or be transported out of the roller housing by a transport device and loaded onto a transport vehicle, so as to be able to be supplied to a recycling process for producing new roadmaking material. However, the milled material may also be prepared as early as the milling process.
A front-loader road milling machine comprising a milling roller arranged in a roller housing is known for example from DE 198 14 053 A1. The hold-down device arranged upstream of the milling roller in the working direction is height-adjustable with respect to the ground. For height-adjusting the hold-down device, a piston-cylinder arrangement fixed to the machine frame is provided. If required, however, the hold-down device may merely be raised, and not pressed down.
Self-propelled ground milling machines of the aforementioned construction are also disclosed in U.S. Pat. No. 4,221,434 and US 2013/0234495 A1.
DE 10 2012 012 397 A1 discloses a road milling machine which has a device for height-adjusting the stripper device or edge protector. While the road milling machine is advancing, the stripper device or edge protector is located in a floating position, in such a way that the stripper device or edge protector is positioned on the ground. Preferably, in the floating position, the stripper device or edge protector is positioned with the weight thereof on the ground. However, a predetermined force greater than the weight may also be applied to the stripper device or edge protector by a device for height-adjusting the stripper device or edge protector.
DE 10 2012 012 397 A1 deals with the problem that the stripper device or edge protector can strike against obstacles as the road milling machine advances. The device for raising and lowering the stripper device or edge protector therefore has a measurement unit which detects horizontal forces acting on the stripper device or edge protector. If the horizontal force component is greater than a predetermined threshold, the stripper device or edge protector is raised. This provides for the stripper device or edge protector to avoid obstacles.
The stripper device arranged downstream of the milling roller is generally not configured to follow irregularities in the ground, since the milled track downstream of the milling roller is largely planar. By contrast, the hold-down device arranged upstream of the milling roller is configured to follow irregularities. Therefore, the hold-down device generally has runners formed as expendable parts, in such a way that the hold-down device can slide along over irregularities (US 2013/0234495 A1).
In practice, in the operation of a working roller the problem arises that, in spite of the hold-down device positioned on the traffic surface, larger fragments known as clods may break off from the traffic surface as a result of an inhomogeneous condition or inhomogeneous construction of the traffic surface. Clod formation results in irregularities in the milling process, since the road material is not removed continuously by the milling roller inside the milling roller housing. One possible result of clod formation is that the broken-off material is displaced upstream of the milling roller and thus is not supplied to the milling process, in such a way that the substrate is no longer worked on in a continuous manner. On the other hand, if they arrive in the interior of the milling roller, large fragments may be captured by and accelerated by the milling roller, and thus introduce undesirable forces into the milling roller or the milling roller housing. This can lead to damage to the milling roller or the machine. Furthermore, there is the risk that these large fragments will not be comminuted sufficiently inside the milling roller housing, and therefore milled material will not have the desired particle size distribution at the end of the process. This is disadvantageous in particular if the milled material is intended to be conditioned and used for remaking the road. Furthermore, large fragments can impede the process of transporting the milled material away by the transport device, since they can lead to material build-ups inside the transport channel. If the material jams at the transfer point from the transport device to the transport vehicle, this can disrupt the material discharge. As a result, pieces of the milled material may be deflected in undesired directions, and the material transfer can thus be impeded.